The title compounds 2 and 3 are of great importance in terms of bonding theory. They were synthesized from the readily accessible precursor 1. The 4nπ species 2 allows comparison of its bonding with that in analogous -, -, and annulenes. Compound 3 is the first doubly bridged semibullvalene; the degenerate Cope rearrangement is accelerated by the bridges (E CO2Et).
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